1. Field of the Invention
The present invention relates to a method of manufacturing a nonvolatile semiconductor memory device comprising gate electrode which is in an electrically floating state.
2. Description of the Related Art
In a nonvolatile semiconductor memory device, the capability of retaining the written information, i.e., written charge, is an important factor in evaluating the reliability of the memory device. The charge retaining capability depends mainly on two factors. One of the factors is the quality of the oxide film covering the floating gate. If the quality of the oxide film is poor, the charge retaining capability of the memory device is deteriorated. Mobile ions such as ions of Na, K, and Li constitute another factor. Intrusion of the mobile ions from the outside can be prevented by using a BPSG film and a PSG film as the insulation film interposed between wiring layers and as a passivation film, respectively, because the mobile ions are gettered by the phosphorus contained in the BPSG film and the PSG film. However, it is difficult to prevent the intrusion of the mobile ions completely. In the actual nonvolatile semiconductor memory device, the reliability of the memory device is restricted by the mobile ions. The mobile ions intrude into the memory device through mainly the wiring contact. In an integrated circuit which is made finer and finer nowadays, the aspect ratio of the contact hole is on a sharp increase. The high aspect ratio brings about deterioration in the step coverage of the aluminum wiring and the passivation film.
FIG. 1 shows a nonvolatile semiconductor memory device manufactured by the conventional method. As seen from the drawing, the memory device comprises a semiconductor substrate 101, a first insulation film 102, a floating gate 103, a second insulation film 104, a control gate 105, a source or drain region 106, a third insulation film 107, a fourth insulation film (BPSG film) 108, a contact hole 109, a wiring layer 110, and an insulation film (PSG film) 111 containing phosphorus. These films and layers are successively formed on the semiconductor substrate 101 using a patterning technique, etc.
In the conventional nonvolatile semiconductor memory device shown in FIG. 1, a high aspect ratio of the contact hole 109 brings about deterioration in the step coverage of the wiring layer 110 and the passivation film 111 at the portions facing the contact hole 109, with the result that the wiring layer 110 and the passivation film 111 are made very thin at the portions facing the contact hole 109. The mobile ions intrude through the thin portions of the wiring layer 110 and the passivation film 111 and migrate through the oxide film 107 so as to reach the floating gate 103 and, thus, deteriorate the charge retaining capability of the memory device.